1. Field of the Invention
The present invention relates to a slewing stop control apparatus and method for automatically stopping a slewing structure in a slewing type working machine such as a crane at a predetermined slewing angular position.
2. Description of the Related Art
For a slewing type working machine having a slewing body, such as a crane, there is conventionally developed an apparatus and a method for automatically starting a braking of the slewing body when the slewing body is closed to a certain slewing angular position to safely stop it at a certain slewing angular position.
For example, in Japanese Unexamined Patent Application Publication No. 11-139770, there is described an apparatus, in a crane 2 having a boom 4 as shown in FIG. 10, for automatically stop the slewing of the boom 4. According to this apparatus, the slewing region of the boom 4 is partitioned into a slewing allowed area Aa shown as an open area in FIG. 10 and a no-slewing area Ap shown as a hatched area in FIG. 10, based on the working state (a slewing radius Rn, an overhanging stroke of a outrigger 6, a hoist load, etc.). In other words, there are set a pair of slewing stop angular positions θsr and θsl, which are bounds of the slewing allowed area Aa and the no-slewing area Ap, on right and left sides of the current angular position of the boom 4. The apparatus monitors a remaining slewing angle θr from the current angular position of the boom 4 up to the slewing stop angular position corresponding to the current slewing direction of the boom 4 (the right slewing stop angular position θsr in the example shown in FIG. 10). When the remaining slewing angle θr has been reduced to a certain slewing braking angle, the apparatus automatically begins braking slewing regardless the operation stroke of the stroke lever, and then control the braking so as to stop the boom 4 at the slewing stop angular position.
The slewing braking angle is an angle which is required for stopping the boom 4 at the slewing stop angular position by braking at a certain slewing angular acceleration (a certain slewing deceleration). After the beginning of the braking, an angular speed command (a target slewing angular speed) which is decreased as the boom 4 is closed to the slewing stop angular position is set to keep the target slewing angular acceleration, and a supply flow rate to a slewing hydraulic motor is adjusted. Specifically, there is gradually reduced an opening area of a control valve between the hydraulic motor and a hydraulic pump supplying hydraulic oil thereto.
However, this control apparatus can be prevented from accurately controlling, because of disturbance. The apparatus performs the computation for the slewing stopping control on the premise of no disturbance; therefore, if the slewing structure receives an external force (for example, a wind, or a grave force on the boom 4 when the crane is declined), the boom 4 will be stopped at a position before the expected slewing stop angular position, in spite of the above-mentioned braking control.
The thus stopped boom 4 must be moved again in the direction equal to the previous one, in order to reach the slewing stop angular position; however, in the above conventional control apparatus, any operation of the control lever cannot restart the boom 4. The control apparatus is so designed as to determine the slewing angular velocity command, namely the target slewing angular velocity, based on the remaining slewing angle between the current angular position of the slewing structure and the slewing stop angular position, and then set the opening area of the control valve so as to make the difference between the target slewing angular velocity and the actual slewing angular velocity 0 (zero); therefore, after the external force make the boom 4 be stopped before the slewing stop angular position, any operation of the control lever cannot give a slewing torque larger than the torque by the external force. That is because: the stop of the boom 4 means that the slewing torque given to the slewing structure for the target slewing angular velocity and the torque by the external force is balanced, and the target slewing angular velocity is determined based on the remaining slewing angle regardless the operation stroke of the control lever.
As to this, the above patent application publication discloses a automatic reset switch, which is provided in a cab and pushed to give the operation stroke of the control lever precedence over the angular position of the slewing structure so as to supply the hydraulic oil to the hydraulic motor by a flow rate corresponding to the operation stroke; however, the reset switch cannot allow sufficient safety to be easily ensured. The operation of the reset switch forces the target slewing angular angle to be determined, regardless the remaining slewing angle, only based on the operation stroke of the control lever, thus not allowing the safety to be ensured by automatic control. Accordingly, an excessive operation of the control lever might give the boom 4 an excessive speed to cross the slewing stop angular position (that is, to enter the no-slewing area).